Y. H. Wong et al.: Development of Sn – Cu – Sb alloys for making lead- and bismuth-free pewter
Yew Hoong Wonga,b , S. Ramesha,b , C. Y. Tana , B. Projjalc
a Department
of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia
of Advanced Manufacturing and Material Processing, University of Malaya, Kuala Lumpur, Malaysia
c Southern Steel Berhad, Prai Industrial Estate, Penang, Malaysia
2013 Carl Hanser Verlag, Munich, Germany
www.ijmr.de
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b Centre
Development of Sn–Cu–Sb alloys for making
lead- and bismuth-free pewter
A systematic study on the development of a set of Sn – Cu –
Sb alloys and their characteristics, such as phases evolved,
mechanical properties, physical properties, and microstructures, that are commonly sought for making pewter is presented. Alloys with various nominal compositions of Sn –
Cu (1 – 3 %) – Sb (3 – 6 %) were prepared and they were die
cast for complete characterization. The samples were characterized for hardness, malleability, density, microstructure, and phase identification. The study is expected to help
in selecting the right composition of the alloys for making
pewter with appropriate combination of properties.
Keywords: Pewter; Alloy; Intermetallic
1. Introduction
Generally, pewter alloys traditionally consist of 85 – 99 %
tin (Sn), with the remainder consisting primarily of copper
(Cu) and antimony (Sb) that act as hardeners and an addition of lead (Pb) for lower grades of pewter that have a
bluish tint. Other elements such as silver (Ag) and bismuth
(Bi) are also sometimes used [1, 2]. However, throughout
its long history, pewter has had an almost infinite variety
of compositions, and even today, numerous varieties are
produced. The only common factor in these variations is
the relatively high tin content that is hardened by additions
of other elements. For instance, English pewter is a strictly
controlled alloy, consisting of 92 % of tin, with the balance
comprising of antimony and copper. Significantly, it is free
of lead and bismuth. The modern tin-based pewter is lead
free with 91 % tin, 7 % antimony, and 2 % copper and is often used to replicate colonial dishes [2]; however, the exact
percentages vary among the manufacturers.
Physically, pewter is a bright and shiny alloy that is very
similar, if not identical, in appearance to silver. Like silver,
pewter will also tarnish to a dull gray appearance over time
if left untreated. Pewter is a very malleable alloy, being soft
enough to carve with hand tools, and it also takes good impressions from punches or presses. However, owing to this
inherent softness and malleability, pewter cannot be used
to make tools. Pewter has a low melting point of around
225 – 240 8C depending on the exact mixture of metals. Duplication by casting will give excellent results [2].
In the Middle Ages, pewter was used for making plates,
drinking vessels, and other tableware, but its usage was not
exclusively confined to these. However, the pewter used
had high lead content, as high as 30 %, which would leach
out upon contact with acidic foods. Lead poisoning can result in death, but it is not a quick process, on the contrary,
it is a process of slow accumulation of toxins over time.
Pewter alloyed with lead was a popular alloy for cooking
and eating utensils and ornamental casting in colonial
America [2]. Modern pewter alloys are mainly used in
decorative articles, such as collectible statuettes, figurines,
replica coins, and pendants. Pewter is seen to be gaining a
renaissance worldwide. Consumers and craftsmen have rediscovered the beauty and practical function of fine pewter
[3]. Moreover, pewter alloys are also used for soldering
functional articles [4, 5].
Health hazards of lead have led modern pewter to contain
little or no lead [6], leading to its replacement with antimony. Older pewters with higher lead content are heavier,
tarnish faster, and oxidation gives them a darker silver-gray
color that is usually undesirable.
Bismuth is unknown to be toxic when compared to its
periodic table neighbors, such as lead, antimony, and polonium. However, some of the bismuth compounds, bismuth
chloride for instance, are known to be toxic owing to its
corrosive acidity and should be handled with care. As with
lead, overexposure to bismuth can result in the formation
of a black deposit on the gingiva, which is located between
the gum and the tooth. The black deposit can be clearly visible as a linear mark on the gingival surface. This gingival
line is also known as a bismuth line, and it is often the first
sign of bismuth poisoning [7].
Owing to the major drawbacks of having lead and bismuth in pewter, lead-free and bismuth-free pewter have to
be developed. The typical properties of tin, namely, low
melting point, absence of work hardening, high malleability, excellent surface appearance, and high oxidation resistance are the major drivers towards the development of this
group of alloys. However, its manufacture involves considerable human skill and any related scientific studies or reports are scarce. Therefore, in this paper, we investigate
the properties of lead-free and bismuth-free tin-based alloys
for making pewters. The present work was undertaken to
bridge the yawning knowledge gap existing in this area. A
systematic study is presented here on the development of a
set of Sn – Cu – Sb alloys and their characteristics, such as
phases evolved, mechanical properties, physical properties,
and microstructures that are commonly sought for making
lead- and bismuth-free pewter.
1
Y. H. Wong et al.: Development of Sn – Cu – Sb alloys for making lead- and bismuth-free pewter
2. Experimental procedures
2013 Carl Hanser Verlag, Munich, Germany
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2.1. Fabrication of samples
First, a half-split mild-steel mould was prepared for pewter
casting. Tin ingot of 99.9 % purity (supplied by Malaysia
Smelting Corporation, Malaysia), copper powder of 99 %
assay (supplied by Fluka, Germany), and an antimony rod
of 99.9 % purity (supplied by Good Fellow, UK), were procured as the raw materials for pewter casting. Compositions
of the Sn – Cu (1 – 3 %) – Sb (3 – 6 %) were prepared for the
study (Table 1). The prepared composition was then placed
in a crucible for melting at 300 8C in the furnace. After the
alloying was completed in the furnace, the pewter alloy
sample was poured out carefully from the crucible into the
mould cavity. After the molten pewter alloy was poured
into the mould cavity, it was allowed to cool and solidify
in the mould. Finally, the solid pewter alloy sample was
formed and removed from the mould.
2.2. Characterization
Density measurements were conducted by using a digital
density meter. Microhardness of the samples was measured
with a Shimadzu Vickers microhardness tester. Surface preparation (grinding and polishing) of the sample was necessary to ensure a well-defined indentation that could be accurately measured. For each test, a small diamond indenter
having a pyramidal geometry was forced into the specimen
with 0.3 kgf load (P) and a constant loading time of 10 s
was applied for all the measurements. The resulting impression was observed under a microscope and measured. After
the indentation, the lengths of the diagonal of the indent, d1
and d2, were measured. An average of d1 and d2 yields d.
Three measurements were performed on each sample and
the average values were taken. Vickers hardness was obtained by dividing the kgf load by the millimeter square
area of the indentation. The Vickers microhardness value
was calculated with the help of the trigonometry function,
leading to its simplified equation (1):
Vickers Hardness; HV ¼ 1:854P=d 2
were then compressed and the deformation at various loads
was recorded. Compressive stress and strain were calculated and plotted as a stress–strain diagram that was used
to determine the compressive strength and malleability.
An automated Bruker DX 8 X-ray diffractometer (XRD)
was used for phase identification and analysis of the pewter
samples. XRD patterns were scanned in steps of 0.03428
(2h) with a fixed counting time of 1 second. The scan was
started from 08 to 1808 (2h) or 08 to 908 (h), by using copper
K-alpha radiation (Cu-Ka) with a wavelength (k) of
1.5406 nm as the X-ray source.
For microstructure analysis, the samples were initially
ground by using successively finer abrasive silicon carbide
(SiC) papers, followed by polishing by using 1 lm and
0.5 lm alumina (Al2O3) powder. The samples were then
cleaned in an ultrasonic bath for 3 min. Subsequently, the
samples were etched by using ferric chloride (FeCl3) solution of a concentration of 1.0 M for 5 s at room temperature.
Etched samples were then placed inside the chamber of a
Zeiss Supra 35 VP field emission scanning electron microscope (FESEM) for characterization.
3. Results and discussion
3.1. Density
The densities of the samples are summarized and compared as bar graphs in Fig. 1. From Fig. 1, it is observed
that the addition of Cu increases the density of the Sn –
Cu alloy. The coordination numbers in the crystal lattice
increase with an increase in the amount of Cu added.
Compared to Sn – Cu alloys, the addition of Sb replacing
Cu lowers the density. Sb helps in lowering the density of
pewter alloys, thus offering lighter weight. The pewter alloys, containing 6 wt.% of impurities, have approximately
the same coordination number, thus exhibiting almost the
same density.
ð1Þ
Compression testing was conducted by using a force that
compressed the specimen along the direction of loading by
using a Shimadzu universal testing machine (UTM). The
samples prepared were placed in a flat surface so that the
application of uniaxial loads was uniform. The samples
Table 1. Pewter compositions for the binary Sn – Cu and ternary
Sn – Sb – Cu systems.
2
Sample Identity
wt.% Sn
wt.% Cu
wt.% Sb
100Sn
99Sn-1Cu
98Sn-2Cu
97Sn-3Cu
94Sn-3Sb-3Cu
94Sn-4Sb-3Cu
94Sn-5Sb-3Cu
94Sn-6Sb
100
99
98
97
94
94
94
94
0
1
2
3
3
2
1
0
0
0
0
0
3
4
5
6
Fig. 1. Effects of various compositions on the densities of pewter alloys. Error bars define the maximum and minimum densities obtained.
Y. H. Wong et al.: Development of Sn – Cu – Sb alloys for making lead- and bismuth-free pewter
Microhardnesses of the samples are shown in Fig. 2. The
hardness of Sn – Cu pewter alloys increase with the addition
of Cu. Atomic radii of Cu and Sn are 0.128 nm and
0.151 nm respectively [8]. This indicates that Cu atoms are
smaller than Sn atoms, thus allowing the Cu atoms to be
fitted interstitially into the Sn atoms during the solid solution process. When the Cu atoms are mixed homogeneously
in the solid solution with Sn, stress fields are created around
each Cu atom. These stress fields interact with the dislocations and make the movement of Cu more difficult [9, 10].
With an increase in the dislocations generated in the alloys,
their hardness can be increased.
Figure 2 also illustrates that the presence of Sb in the
ternary Sn – Sb – Cu alloys enhances the hardness of the
pewter remarkably. The highest hardness value is achieved
with the addition of 3 % Cu to the Sn – Cu binary alloys.
The addition of Sb to the alloys will further increase
their hardness. The highest hardness value of 1.55 GPa
(157.82 kgf mm–2) is achieved by the ternary pewter alloy
with its composition of 94 % Sn, 3 % Sb, and 3 % Cu. Sn –
Sb – Cu and Sn – Sb alloys are harder than Sn – Cu alloys
owing to the presence of intermetallic phases in their microstructures [10], such as Cu6Sn5 and SbSn. Moreover, because of the higher concentration of alloying elements in
Sn – Cu – Sb ternary alloys and Sn – Sb binary alloys
(6 wt.%) when compared to the Sn – Cu binary alloys
(3 wt.%), the pewter is stronger and harder for the same reason as discussed before. Unlike the effect of copper addition, the addition of Sb only marginally decreases the hardness. This is because the solubility of Sb in Sn decreases
with the addition of Sb to Sn.
3.3. Malleability
Figure 3 compares the malleability of the pewter alloys estimated by means of compression testing. Introduction of
Sb and Cu into the alloy gradually reduces malleability. As
discussed earlier, owing to the motions of the dislocations,
the alloying elements (Sb and Cu) go into solid solution
and impose lattice strains on the surrounding host Sn atoms
making them harder, less malleable, and more difficult to
deform. In addition, the higher concentration of alloying
elements in the Sn – Sb and Sn – Sb – Cu alloys (6 wt.%)
make them less malleable than the Sn – Cu alloys (3 wt.%).
Apart from that, the formation of intermetallic compounds
in the Sn – Sb – Cu and Sn – Sb alloys can be responsible
for their hardening and strengthening [9, 11].
Fig. 2. Influence of Sb and Cu as alloying elements on the microhardness of pewter alloys. Error bars define the maximum and minimum
microhardness obtained.
2013 Carl Hanser Verlag, Munich, Germany
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3.2. Microhardness
Fig. 3. Stress–strain curves for Sn – Sb – Cu
pewter alloys. Inset shows the close-up
stress–strain curves for Sn – Sb – Cu pewter
alloys.
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Y. H. Wong et al.: Development of Sn – Cu – Sb alloys for making lead- and bismuth-free pewter
Figure 4a and b shows the XRD patterns for the Sn – Cu and
Sn – Cu – Sb, alloys, respectively, at different compositions.
In the binary alloys (Sn – Cu), only Sn and Cu were found
(no intermetallic formation). However, in the Sn – Sb – Cu
and Sn – Sb alloys, formation of intermediate phases
(Cu6Sn5 and SbSn) was evidenced. Sn – Sb – Cu ternary alloys and Sn – Sb binary alloys have higher hardness values
and less malleability than the Sn – Cu binary alloys. Thus,
stronger alloys can be produced. In this study, Cu6Sn5 and
SbSn play an essential role in increasing the strength of the
alloys. The intermetallic phases formed in the process help
in strengthening the alloys [9, 11].
3.5. Analysis of microstructures
Pure Sn microstructure is shown in Fig. 5a, while Sn – Cu
alloy microstructures with varying Cu contents are shown
in Fig. 5b – d. From these micrographs, it can be seen that
the grain size reduces with increasing Cu content. Therefore, the higher Cu content in the Sn – Cu alloy makes it
stronger. The microstructures of Sn – Sb – Cu and Sn – Sb alloys are shown in Fig. 5e – h. The grain sizes are finer and
some near-square or triangle-like shapes are observed,
which may be the SbSn phase [12]. In addition, coarse dendrites indicating Sn-rich solid solution caused by the rapid
solidifications of the alloys can be seen [12].
4. Conclusions
With the addition of alloying elements, mainly Cu and Sb,
the physical and mechanical properties of the pewter were
enhanced. Alloying elements Cu and Sb improve hardness
and hence are expected to improve the wear resistance as
well. Increasing both Cu and Sb in the alloys made them
less malleable, and thus enhanced their compressive
strengths. Addition of Cu increased the alloy density while
Fig. 4. XRD patterns for (a) Sn – Cu(1 – 3 %) and (b) Sn – Cu(1 – 3 %) –
Sb(3 – 6 %).
2013 Carl Hanser Verlag, Munich, Germany
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Not for use in internet or intranet sites. Not for electronic distribution.
3.4. XRD analysis
Fig. 5. FESEM micrographs for: (a) Pure Sn (b) Sn-1Cu (c) Sn-2Cu (d) Sn-3Cu (e) Sn-3Sb-3Cu (f) Sn-4Sb-2Cu (g) Sn-5Sb-1Cu (h) Sn-6Sb.
4
the addition of Sb made it less dense. The addition of Sb increases compressive strength and makes the alloy lighter in
weight. Formation of intermetallic compounds was detected in the alloys, which may help to enhance their mechanical properties. Furthermore, pewter alloys produced
from various combinations of Sn, Cu, and Sb exhibited a
shiny surface with a high degree of brightness. In summary,
through the appropriate alloying of Sn-based alloys with Cu
and Sb, it is possible to choose the right alloy composition
for making pewter that has the right combination of properties.
The authors would like to acknowledge University of Malaya for the
financial support through UMRG programme (RP013D-13AET) and
PRPUM (CG022-2013).
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Bibliography
DOI 10.3139/146.111011
Int. J. Mater. Res. (formerly Z. Metallkd.)
104 (2013) E; page 1 – 5
# Carl Hanser Verlag GmbH & Co. KG
ISSN 1862-5282
Correspondence address
Dr. Yew Hoong Wong
Department of Mechanical Engineering
Faculty of Engineering
University of Malaya
50603 Kuala Lumpur
Malaysia
Tel.: +603-7967 7022 ext 2654
Fax: +603-7967 5317
E-mail: [email protected]
You will find the article and additional material by entering the document number MK111011 on our website at
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2013 Carl Hanser Verlag, Munich, Germany
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Not for use in internet or intranet sites. Not for electronic distribution.
Y. H. Wong et al.: Development of Sn – Cu – Sb alloys for making lead- and bismuth-free pewter
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